Power-measuring bridge



March 25, 1947. E. l.. GlNzToN POWER-MEASURING BRIDGE Filed NOV. 3, 1943 v j ma o.. o 32m mo omo I Y RG. m. m.. m .50%, vR m mA Patented Mar. 25, 1947 2,417,820 POWER-MEASURING BRIDGE Edward L. Ginzton, Garden City,- N. Y., assignor to Sperry Gyroscope Company, Inc., a corporation of New Yorlrl Application November 3, 1943, Serial No. 508,804 2 Claims. (Cl. 171-95) Serial No. 429,508 for High frequency power measuring device, :tiled February 4, 1942, in the names of W. W. Hansen, J. R.. Woodyard and E. L. Ginzton. Thepresent invention may be considered an improvement over that disclosed in said ccpendlng application.

An object of the invention is to provide an improved power bridge having a hot wire element, the resistance of which is maintained constant.

An object of the invention is to provide a Wollaston wire type .of balanced bridge by means of which low power measurements may be made aci curately.

Still another object of the invention is to provide apparatus for measuring, alternating cur-l rent power in a direct current bridge by the substitution method.

Still another object is to provide a direct current power bridge with means for absorbing excess power and maintaining constant power dissipation in the hot wire unit as alternating current power input is varied.

Other and further objects and advantages will become apparent as the description proceeds.

In carrying out the invention in its preferred form, a hot wire element is employed in which the alternating current power to be measured is dissipated. A direct-current source is utilized for heating the wire. Meansv are provided for ascertaining the direct-current power consumption both when alternating current is applied and when alternating current` power is absent and formaintaining the total power dissipation of the hot wire constant, in order that the dilerence in the two values of direct-current power will represent the alternating-current power measured. In order to maintain the power dissipation constant, means are provided for measuring the resistance of the hot wire and holding it at a constant value. To this end, the hot wire is made one arm of a resistance bridge. In order to make it unnecessary to detect small variations in direct current power input to the bridge directly When the alternating current power varies, the apparatus is arranged for a xed power output from a direct current source. As the alternating current power input to the hot wire unit varies, the

2 requisite variations in direct current power input tothe hot wire unit for maintaining the hot wire resistance constant are accomplished by absorbing the excess power in calibrated attenuator units which provide a'constant impedance across the direct current source. The attenuator units are adjustable to absorb varying amounts of power from the direct current source without alecting the total power absorbed therefrom and may be so calibrated that the alternating current power may be read directly from the settings of the attenuator units.

A better understanding of the invention will be afforded by lthe following detailed description, consideredin conjunction with the accompanying drawing.

The form of the invention shown for the sake of illustration in'the drawing comprises a direct current bridge Il, a source of direct current I2,

such asa single dry cell for energizing the bridge, means for measuring the direct current input to the bridge, such as a milliammeter I3, and attenuator apparatus i4 for absorbing excess direct current power input to the bridge. For initially adjusting the direct current power input to the bridge to give a zero reading when there is no alternating current input, coarse and ne series rheostats I5 and I6 are preferably provided.

The bridge II comprises four resistance elements Il, I8, I9 and 2| in series parallel, and a null indicating device, such as a galvanometer 22 connected as a diagonal arm of the bridge between conjugate points 23 and 24. Two other conjugate points 25 and 26 are connected in series with the direct current input source I 2, the instrument I3, and the adjusting rheostats I5 and I6 as in a. conventional Wheatstone bridge. One of the bridge arms I9 may take the form of a hot wire or Wollaston wire unit of a type heretofore known, having means such as a length of concentric transmission line 53, represented merely schematically by a parallel conductor pair with an interposed D.C. isolating condenser 20 for supplying alternating current power, more particularly, high frequency or microwave radio frequency energy from a source 55 to the wire I9 simultaneously with the passage of direct current through the Wollaston wire unit. A hotwire holder of any suitable type may be employed,

e. g., one such as disclosed in one of thecopending applications of'Arthur E. Harrison, Serial No. 509,889,- filed November 11, 1943, and Hugh E.. Webber, Serial, No. 513,164, led December 6, 1943, Serial No. 514,109, filed December 13, 1943, Serial No. 514,111, .dled December 13, 1943.

The resistors I1, I 8 and 2| are preferably standard resistors so constructed as to maintain substantially constant resistance. The resistance values are so chosen that the bridge |I is balanced when the Wollaston wire unit i9 has a resistance corresponding to the maximum safe value of power input and heating. For maximum accuracy, the resistances of the four arms of the bridge are preferably equal.

The power absorption apparatus Il comprises one or more calibrated attenuators, for example, decibel attenuators. For example, there may be a pair of conventional T pads or T-type Daven attenuators 21 and 28 arranged for coarse and intermediate steps of adjustment, respectively, and a third attenuator 29 designed for exceedingly fine steps of adjustment. For example, the attenuator 21 may have a range from zero to thirty decibels in steps of one decibel; the attenuator 28 may` have a range from zero to three decibels in steps of one-tenth decibel; and the attenuator 29 may have a range ci zero to onetenth decibel in steps of one-hundredth decibel. AS is well known these T pads or attenuators contain variable resistors arranged to attenuate a signal without changing the input impedance of the attenuator.

It Will be understood, of course, that the values stated are illustrative, and the invention is not limited to the precise ranges of attenuation suggested. That is, the accuracy desired in the measurement and the minimum value of alternating current power to be measured will determine the actual dimensions of the attenuators. The attenuators 21 and 28 may be of a type heretofore known. The ne adjustment attenuator 29 may also be of the T-type with additional elements for increasing the neness of adjustment.

As shown, the attenuator 29 comprises a pair of fixed-value series resistors 3| and 32, a pair of resistors 33 and 34 connected in series in a circuit shunting the vbridge and a pair of parallel series resistors 35 and 36 rconnected as a 'series pair shunting the resistors 3| and 32, which are in series with the bridge i? I.

One of the shunt resistors 34 and one of the parallel-series 36 are adjustable in value and they are preferably provided with mechanically connected adjusting arms schematically represented by arrows 31 and 38 arranged to increase the resistance of one rheostat while decreasing the resistance of the other.

In order to avoid variations in direct current load on the source I2 and consequent variations in the current flowing through the milliammeter I3 when the attenuators are varied in adjustment, the attenuators are preferablymatched to each other and to the bridge II. For, example, if the bridge arms each have a resistance of 125 ohms, the attenuators will be matched thereto so as to present a comparable impedance to the bridge circuit I|. For example, in the case oi' attenuator 29, resistors 3| and 32 in series with bridge are yeach of a value of 125 ohms. The shunt circuit resistors 33 and 34, however, are of substantially greater resistance, for example, 3,000 ohms and 2,000 ohms respectively, and the series resistors 35 and 36 are substantially of less resistance, for example, 3 ohms and 2 ohms respectively, the values stated for the variable resistors 34 and 36 being the maximum values. For an illustrative system of the values stated, the milliammeter I3 may have a range of zero to 10 milliamperes, and the adjusting rheostats I and I6 may have maximum rcsistances of 300 and 20 ohms, respectively. It will be understood, of course, that the invention is not limited to the illustrative values stated, which have been given merely for the purpose oi setting forth illustrative resistance relationships between the elements which have been found to operate satisfactorily.

Since the attenuators 21 and 28 are of the T- type, the upper terminals do not interpose any resistance, and a conductor 39 provides a zero attenuator 21, a conductor 43, the attenuator 28,

a conductor 44, the series resistors 3| and 32 of the attenuator 29 by-passed by the resistors '25 and 36, a conductor 45, the bridge input terminal 26, the bridge II, the bridge input terminal 25, back to the source I2 through a conductor 46.

With this arrangement of the attenuators connected to the bridge I I and the source I2, the load impedance on the source i2 remains constant and the voltage output is therefore unaffected by adjustment of the attenuators because they have a constant input impedance for all adjustment.

Preparatory to making power measurements with the bridge, the alternating current power input to the Wollaston wire unit I9 is cut oft, and the apparatus is adjusted to give a zero power reading. This is done by setting the attenuators 21, 28 and 29 to minimum attenuation and adjusting the resistors I5 and I6 until the galvanometer 22 gives a null deflection, indicating that the bridge is balanced, and the Wollaston wire unit I9 has a predetermined resistance equal to resistances of the standard resistors I'I, I8 and 2|. The reading of the milliammeter I3 may then be noted, While maintaining a fixed power output thereafter. However, if a constant voltage source I2 is employed, this could be unnecessary for the brief interval of time required to make an alternatingl current power measurement.

Since the attenuators 2'I and 28 are not my invention but apparatus available in the market, the internal operation thereof need not be described, and the zero attenuation adjustment Iis obtained by setting these attenuators to the zero points on their scales.

The attenuator 29 is set for minimum attenuation by setting the resistor-adjusting handles 31 and 38 to the values making the resistance 34 maximum and the resistor 36 minimum.

Before applying alternating-current power to the Wollaston wire unit i9, the attenuators 21,

28 and 29 are set for maximum attenuation to indicating that the dissipation in the Wollaston wire unit I9 and the resistance thereof have been restored to their predetermined standard value.

Y Since the resistance of the wire I9 depends upon its temperature, which in turn depends upon the total dissipation of energy therewithin regardless of whether the source of the energy anaso is alternating or direct current, the alternating current power measured will be the diierence between the direct current power input tor-the wire -before the alternating current power is applied and afterwards. Since the apparatus is operated with a power input from the source l2 constant, the difference in direct current power with and Without the .application of alternating current power will be represented by the loss of direct current power in the attenuators 21, 28 and 29, and the alternating current power may -therefore be measured by observing the attenuunit alternating current, the power of which is to be measured. a direct-current source, the output of which may be maintained constant. adjustable calibrated constant input impedance attenuator means, and connections from the direct-current source to said bridge and to said calibrated attenuator means for causing the at-A tenuator means to absorb power according to the setting of the attenuator means, whereby the to tal power input to the hot wire unit may be maintained constant with variations in alternating-current input power -by varying the adjustment of the attenuator means to absorb excess direct current power while retaining constant the eiective impedance load for said direct current source. Y

2. Apparatus for measuring alternating-current power, comprising a direct-current power bridge, one arm of which comprises ahot wire 5 to maintain constant dissipation of the Wollas- 2 ton wire unit I9. It will be understood, of course, that the attenuators 21, 28 and 29 may be calibrated in terms of alternating current power of voltage vand resistance are used in the other parts of the apparatus.

In accordance with the provisions of the patent statutes the principle of operation of the invention has been described together with the apparatus now believed to represent the best embodiment thereof, but it is to be understood that the apparatus shown and described is only illustrative and that the invention may be carried out by other arrangements.

What is claimed is:

l. Apparatus for measurinar alternating-current power, comprising a, direct-current power bridge, one arm of which comprises'a hot wire unit with means for supplying to .the hot wire 45 1,791,563

vmeasured instead of attenuation, if xed values 30 unit with means for supplying to the hot wire` unit alternating current, the power of which is to be measured, a direct current source, and calibrated constant input impedance attenuator means connected between said direct current source and said bridge, said attenuator means having a calibrated adjustment to vary the direct current power absorbed by said bridge while maintaining constant the total power drawn from said direct current source for measuring the alternating power absorbed by said unit.

EDWARD r.. GrNzToN.

REFERENCES CITED' The following references are of record in the fue of thispatent:

UNITED STATES PATENTS Number Name Date 40 1,121,835 Hiatt T.. Dec. 22, 1914 1,957,454 Gebhard -1---- May 8, 1934 2,140,364 Lee Dec. 13, 1938 2,005,986 Behr June 25, 1935 1,590,420 Chubb June 29,- 1926 H0m";.:.:...' Feb. 10, 1931 

